Sorghum [Sorghum bicolor (L.) Moench] is commonly grown in water-limited environments throughout the world.  Hence, approaches to increased productivity of sorghum need to focus on improvements in crop water use.  One approach is to have conservative water use early in the growing season so that it is available for use later in the season when drought is most likely to develop.  Expression of a maximum-transpiration rate under high, midday vapor pressure deficit (VPD) conditions can result in such water conservation.  The objective of this study was to evaluate nine sorghum genotypes for expression of the maximum-transpiration trait under both controlled chamber and field conditions.  In the growth chamber, plant transpiration rate (TR) was measured over a range of imposed VPD to provide direct measure of limited transpiration under high VPD.  In the field, stomatal conductance (g_s) was measured over the daily cycle, which resulted in a range of ambient VPD.  A decrease in g_sunder high VPD was evidence of the maximum-transpiration trait. Generally, there was a correspondence within genotypes between the controlled chamber and the field results, although there were exceptions. The genotypes expressing the maximum-transpiration trait in both tests are candidates for evaluating potential benefits of this trait in improving yield.

Key words; vapor pressure deficit (VPD), stomatal conductance (g_s), transpiration rate (TR)